The invention is based on a priority application EP07300772.6 which is hereby incorporated by reference.
The invention relates to a method for interleaving bits according to the preamble of claim 1, a base station according to the preamble of claim 6, a user terminal according to the preamble of claim 7 and a communication network according to the preamble of claim 8.
The transmission of data over communication channels, especially over radio channels in a mobile communication system or in a wireless data system, is often error-prone, as such effects like interference, fading, shadowing or noise deteriorate the transmission signal and thus lead to bit errors.
As such bit errors tend to have a cumulative occurrence e.g. over transmission time or bandwidth, bits that have to be transmitted over a communication channel are preferably interleaved in a so-called interleaver in order to achieve a statistical distribution of bit errors.
After the channel encoding at the transmitter by e.g. a Turbo encoder, an interleaver is used to rearrange the bits within the transmitted codewords. On the receiver side, a deinterleaver performs the inverse rearrangement on the received codewords before the actual channel decoding. This procedure transforms channel induced burst errors into statistically independent errors and is vital in mobile radio communication systems with fading channels.
In common interleaver implementations, a codeword that has to be transmitted over a communication channel is first segmented into k≧1 blocks. Each of the blocks is interleaved separately in an interleaver, i.e. the number of blocks corresponds to the number of interleaving cycles that have to be performed. An interleaver basically comprises a matrix into which the information bits are written column-wise and then read out row-wise. The number of columns N of the individual interleavers is fixed. Between writing and reading, the columns may be permuted according to a given permutation rule. The number of blocks k and the number of matrix rows Ri, i=1 . . . k, must be adjusted to the codeword size. The number of blocks k is fixed or takes on one of only a few predefined values. In the latter case it is usually found by table look-up. For performance reasons it is desirable to have individual interleavers with N≈Ri, i.e. the number of rows should be approximately equal to the number of columns. With these individual interleaver dimensions, burst errors within a codeword introduced by e.g. a fading channel are transformed into statistically independent errors in the best possible way. This is very important because forward error correction channel encoding, e.g. Turbo encoding, usually requires statistically distributed errors to leverage its full potentials.
As the deinterleaver performs the inverse operation, it follows directly from the interleaver.
In the 3GPP Evolved UTRAN (3GPP=Third Generation Partnership Project, UTRAN=Universal Mobile Telecommunication System Radio Access Network), also known as Long Term Evolution (LTE), the number of data symbols within the codewords transmitted from a so-called NodeB, i.e. a base station, to the so-called user equipment (UE), i.e. a user terminal, may change with a granularity of 1 symbol from codeword to codeword, depending on cell traffic, signaling overhead, radio conditions, etc. As each data symbol represents a number M of information bits, the granularity of the information bits within a codeword is M, e.g. M=2 for QPSK modulation (QPSK=Quadrature Phase Shift Keying).
An interleaver that is appropriate for usage within 3GPP Evolved UTRAN must offer a high flexibility towards the codeword size.
One solution for a 3GPP Evolved UTRAN compliant interleaver could be the UMTS (UMTS=Universal Mobile Telecommunication System) HSDPA (HSDPA=High Speed Downlink Packet Access) interleaver defined in the technical specification 3GPP TS 25.212, V7.2.0 (2006 June) in chapter 4.5.6 with the restriction to only few codeword sizes, as the interleavers work with a fixed number of columns N=30, a fixed number of rows Ri=R=32 and a number of blocks kε{1, . . . , 15}.
For a high codeword size flexibility and a fine codeword size granularity, as used in the 3GPP Evolved UTRAN, a restricted interleaver parameter set, as e.g. used for the UMTS HSDPA interleaver described above, will lead with a high probability to individual interleavers that are not close to the desirable square form (N≈Ri), but exhibit a rather large difference between the number of columns N and rows Ri.